1. Field of the Invention
The present invention relates to a plant growth media comprising a polyurethane gel composition and an article containing same. The invention also relates to a method of utilizing the plant growth media.
2. Description of the Prior Art
Polyurethane gel production is known. The Windemuth et al U.S. Pat. No. 2,948,691 discloses that isocyanate modified polyglycolethers, especially those having a molecular weight higher than 2,000, are soluble in water. These polymers are prepared by reacting polyglycolethers having at least two terminal hydroxyl groups with either mono- or polyfunctional aliphatic or aromatic isocyanates. Aqueous solutions prepared with these materials are not stable because of the high reactivity of the isocyanate group in water. Thickening or gel formation is said to occur with increasing molecular weight within an interval of some minutes depending on the solid content of the solution.
For example, a solution of 4 percent of an isocyanate modified polyglycolether prepared from a polyglycolether of the molecular weight of 4,030, which is obtained by the addition of ethylene oxide to trimethylolpropane, produces a gelatinous mass. In another example an isocyanate modified polyglycolether containing 3.2 percent of NCO groups, was obtained from a polyglycolether based on the addition of ethylene oxide to trimethylolpropane and toluene diisocyanate. Upon stirring the product into water it solidified into an insoluble gel with the evolution of carbon dioxide which was perceived by the formation of bubbles in the gel, especially when large quantities of isocyanate are used. Windemuth discloses this gel-forming reaction as being adapted to the thickening of aqueous solutions or emulsions. Windemuth's reaction products of the polyglycolether and the isocyanate are summarized as being used for producing plasticizers, lubricants, plastics, spongy materials, gel formers, thickening agents, auxiliaries in textile industry and the like, with no teaching or suggestion of plant growth media use.
In the Asao et al U.S. Pat. No. 3,719,050 a polyurethane prepolymer having terminal isocyanate groups is reacted with water in the soil to gel the polyurethane prepolymer and solidify the soil. The elastomer formed is said to have excellent strength and to strongly adhere to the soil to improve the soil properties.
The polyurethane prepolymer of Asao et al is synthesized from 2 to 8 moles of polyisocyanate per mole of a polyalkylene oxide compound. The alkylene oxides employed include ethylene oxide (EO), propylene oxide (PO), butylene oxide (BO), as well as styrene oxide and epichlorohydrin. In preferred embodiments the prepolymer is obtained as a reaction product of an ethylene oxide-propylene oxide copolymer having an ethylene oxide unit and propylene oxide unit in a ratio of from 90/10 to 70/30, which is reacted with an isocyanate compound having at least two isocyanate groups.
The polyurethane prepolymer is liquid or pasty at room temperature, and when added to 97 to 85 percent by weight of water, it readily dissolves, thereby forming a transparent solution, or is dispersed homogeneously therein to form an emulsion. The aqueous solution or emulsion is converted in a period of 20 seconds to two hours to a water-insoluble water-containing plastic material having a high elasticity. The polymerization of the urethane prepolymer by the cross-linking reaction with water produces carbon dioxide. Since the carbon dioxide gas produced is dissolved in the water present in the system, the resultant water-containing plastic material is a gel with a slight increase in volume owing to the gas formation.
The only utility of this polyurethane prepolymer is to form a gel used to solidify and stabilize soil. Inorganic materials such as clay, cement and the like can be mixed with the prepolymer and then injected into the soil. The elastomer which forms is said to have an extremely excellent strength and it markedly improves the soil in view of its strong adherence to the soil.
The Harada et al. U.S. Pat. No. 4,118,354 discloses a similar gel produced by dispersing a reaction product obtained by the reaction of a polyisocyanate having at least two isocyanate groups with a polyether. The polyether comprises a plurality of alkylene oxides, 50 to 90 percent by weight of which is ethylene oxide, added at random to a polyalcohol having at least two terminal hydroxyl groups. The alkylene oxides are added in an amount of an average molecular weight within the range of 1,000 to 4,000 per each hydroxyl terminal group of the polyalcohol.
The disclosed utility of these gels is in disposing of various sludges of the bottom of rivers, seas, and sludges removed from organic and inorganic waste liquids and sewage. The polyurethane hydrogels are also employed in disposing of liquid wastes released from breweries, papermills, dye works, tanneries, slaughter houses, etc. Further, aqueous solutions containing noxious metals and radioactive substances are said to be effectively confined within the polyurethane hydrogel to prevent them from being scattered.
In each of the Windemuth et al, Asao et al and Harada et al patent references, there is no discussion of using their gel compositions as a plant growth media.
Other types of polyurethanes have been used to form substrates for plant breeding and cultivation. The polyurethanes disclosed in the patents, however, result in the formation of hydrophilic polyurethane open celled foams and not gels. See U.S. Pat. Nos. 3,706,678, 4,034,508 and 4,035,951. That is, when the prepolymer used in these patents is mixed with water, the large concentration of NCO groups react with water to generate a substantial amount of carbon dioxide (Co.sub.2). Since these compositions are not based on prepolymer compositions which dissolve in water to form a solution prior to reaction, they do not form gels.
3. Objects of the Invention
It is an object of this invention to obtain a polyurethane gel that can act as a plant growth media supplying water to the plants.
It is a further object to produce a plant growth media from a polyurethane gel composition that can be readily made by dissolving a prepolymer containing plant seeds in an aqueous slurry.
It is a further object to obtain a plant growth media from a polyurethane gel composition which is made from a hydrophilic polyurethane prepolymer having terminal NCO groups and being capable of dissolving completely in an aqueous slurry.
These and other objects of the invention will become apparent from the description of this invention hereinafter.